Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

224 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSxA2XASI t ,Y-42Inmass In gasve~ocityinrunsIFilm diffusion becomes/important belowthb veiocih,IIII 1IJIt(FM)r (FA0)2 (FA0)3 (FMLFBed rate ot gas velockyFig. 24. Method to determine the influence of mass transfer on the rate of reaction. FAO. feed rate;V, catalyst volume; V/FAO, space velocity; XA, extent of conversion. From ref. 188.Leffects are becoming important. A second method, illustrated in Fig. 25, is tocompare two series of experimental runs using different volumes of catalyst and aseries of feed rates. At high space velocities, where mass transfer effects are small,the two sets of data will coincide. When film diffusion becomes important, thetwo curves will diverge. The first method is generally more accurate because itcan cover a wider range of conditions and allows easier detection of mass transferlimitations. These can be removed or reduced by any means which increases gasmixing. In practice this may be done by using high linear gas velocities whichproduce a high degree of turbulence. This point is discussed further in the sectioaon reactor design (Sec. 2.2.3). The temperature coefficient of the reaction alsoindicates the presence of mass transfer effects. Diffusion coefficients have a smalltemperature coefficient, so that apparent activation energies of 1-2 kcal . mole-'will be observed when mass transfer is limiting the reaction rate.(b) Pore diffusionWhen mass transfer to the catalyst surface is fast compared to chemical reaction,the reaction rate may still be limited by the rate of diffusion in the catalyst pores.Bulk diffusion (i.e. the same process as for mass transfer to the external surface)occurs when the mean free path between molecular collisions is small comparedto the pore radius. Knudsen diffusion occurs when the mean free path is large